UNIVERSITY    OF    CALIFORNIA     PUBLICATIONS 

IN 

AGRICULTURAL    SCIENCES 

Vol.  4,  No.  2,  pp.  67-97,  1 1  text  figures  May  1,  1919 


TESTS  OF  CHEMICAL  MEANS  FOR  THE 

CONTROL  OF  WEEDS 

REPORT  OF  PROGRESS 


BY 

GEOEGE  P.   GEAY 


INTRODUCTION 

The  cultivation  of  crops  has  been  actually  abandoned  on  hundreds 
of  acres  of  some  of  the  most  fertile  land  of  the  state  and  the  produc- 
tivity of  thousands  of  acres  more  is  rapidly  decreasing  through  the 
increase  and  spread  of  wild  morning-glory  and  Johnson  grass.  These 
weeds  have  defied  practically  all  control  measures  so  that  their  appro- 
priation of  still  more  choice  land  remains  unchecked.  The  mechanical 
control  methods  commonly  used  against  these  two  weeds  are  hand- 
digging  and  clean  culture  for  at  least  a  year.  The  expense  of  either 
of  these  procedures  is  always  great  and  sometimes  there  must  be  added 
the  loss  of  the  use  of  the  land  during  the  process.  The  expense  in- 
volved may  equal  or  exceed  the  value  of  the  land,  but  the  most  serious 
aspect  of  the  matter  is  that  too  often  the  efficiency  of  the  hand-digging 
or  clean  culture  operations  may  fall  below  a  full  hundred  per  cent,  in 
which  case  the  attempt  usually  results  in  a  complete  failure.  Any 
measure  short  of  complete  eradication  does  not,  as  a  rule,  justify 
the  expense. 

The  two  plants  mentioned  are  undoubtedly  foremost  among  the 
weed  pests  of  the  state,  but  all  weeds  exact  a  heavy  tribute  from  the 
tiller  of  the  soil.  Other  noxious  weeds  of  great  economic  importance 
are:  the  so-called  "water-grass"  of  rice  fields,  Bermuda  grass,  wild 
mustard,  wild  radish,  foxtail,  and  thistles.  These  and  others  have 
been  found  very  difficult  to  control  and  under  some  conditions  their 
increase  and  spread  has  been  so  rapid  as  to  make  the  cultivation  of 
the  infested  fields  unprofitable. 


68  University  of  California  Publications  in  Agricultural  Sciences         [Vol.  4 

Certain  of  our  statutes  declare  noxious  weeds  to  be  a  nuisance  and 
empower  the  county  horticultural  commissioners  to  enforce  the  abate- 
ment of  such  nuisances.  The  enforcement  of  these  laws,  however,  is 
very  difficult  and  in  certain  sections  has  not  been  attempted  on  account 
of  the  magnitude  of  the  undertaking,  the  expense  involved,  or  the 
uncertainty  of  success  of  the  known  means  commonly  used  toward 
eradication. 

These  conditions  constitute  a  very  serious  menace  to  the  agriculture 
of  the  state  and  were  fully  discussed  at  a  meeting  of  the  county  horti- 
cultural commissioners  of  California  held  at  Stanford  University, 
July  26,  1915.  The  use  of  chemicals  was  frequently  mentioned  as  a 
possible  solution  of  the  weed  problem,  but  the  fact  was  brought  out 
that  very  little  information  is  available  concerning  their  usefulness 
under  California  conditions.  Following  this  meeting,  Mr.  William 
Wood,  Los  Angeles  County  Horticultural  Commissioner,  took  up  with 
the  director  of  the  agricultural  experiment  station  at  Berkeley  the  mat- 
ter of  an  investigation  of  methods  of  weed  control  by  means  of 
chemicals.  It  was  suggested  that  a  judicious  use  of  chemicals  might 
solve  the  problem  of  the  control  of  those  weeds  which  too  often  survive 
and  increase  in  spite  of  expensive  mechanical  control  measures.  The 
suggested  investigation  was  undertaken  by  the  station  in  the  fall  of 
1915,  and  the  writer  was  assigned  to  the  work. 

A  review  of  the  literature  discloses  the  fact  that  the  use  of  chemicals 
has  simplified  to  some  extent  the  solution  of  the  problem  of  weed 
control  on  both  agricultural  and  non-agricultural  land  in  localities 
other  than  California.*  In  one  case  at  least,  the  use  of  chemicals  for 
the  destruction  of  weeds  has  become  a  common  practice.9  Some  of 
the  experiments  have  demonstrated  the  usefulness  of  chemicals  for 
the  destruction  of  weeds  on  a  large  scale.  Methods  which  have  proved 
efficient  elsewhere  may  also  prove  to  be  efficient  here.  The  soil  and 
climatic  conditions  of  California,  however,  are  so  different  from  those 
of  regions  where  the  chemical  method  has  proved  satisfactory,  that  it 
is  by  no  means  certain  that  this  will  be  the  case.  A  study  of  the 
literature  gave  encouragement  that  weed  pests  might  be  controlled 
under  local  conditions  if  the  conditions  were  carefully  studied  and  the 
treatment  made  accordingly. 

Herbicide  experiments  have  been  made  by  several  of  the  railroad 
companies  operating  in  the  state.  Probably  the  first  of  the  county 
horticultural   commissioners  to   experiment   with    chemicals   for   the 


•   Sec   references  on    |)U^e  1)7. 


1919]  Gray :  Tests  of  Chemical  Means  for  the  Control  of  Weeds  69 

control  of  weeds  was  Mr.  F.  W.  Waite  of  El  Centro.  Other  commis- 
sioners who  have  tried  this  method  are  Mr.  William  Wood  of  Los 
Angeles,  Mr.  A.  A.  Brock  of  Ventura,  and  Mr.  C.  W.  Beers  of  Santa 
Barbara.  Very  few  data  have  been  published  upon  the  results  of 
these  experiments,  but  the  writer  has  secured  much  valuable  informa- 
tion and  confirmatory  evidence  through  correspondence  with  these 
experimenters  and  by  occasional  inspection  of  their  results.  In  gen- 
eral, the  results  were  encouraging,  but  sufficient  progress  had  not  been 
made  to  establish  a  definite  mode  of  procedure. 

Investigations  by  the  Insecticide  and  Fungicide  Laboratory 

The  following  progress  report  describes  the  more  important  experi- 
ments which  have  been  made  during  the  investigation.  A  mass  of 
data  has  been  accumulated  showing  the  effect  of  a  number  of  sub- 
stances on  plant  life  and  on  soils,  some  of  which  have  been  suffi- 
ciently conclusive  to  warrant  the  recommendation  of  control  measures. 
Some  of  the  experiments  have  opened  up  a  very  promising  field  for 
further  investigation,  while  others  have  been  largely  negative  in 
character.  No  attempt  is  made  in  this  account  of  the  experiments 
to  make  definite  recommendations  for  control  measures.  The  prac- 
tical application  of  the  results  is  reserved  for  discussion  in  separate 
publications  on  special  topics.* 


ACKNOWLEDGMENTS 

An  account  of  the  investigations  would  be  incomplete  without 
special  mention  of  the  work  of  two  student  assistants,  Mr.  C.  C. 
Barnum  and  Mr.  T.  M.  Pierce.  Their  assistance  in  the  planning  and 
execution  of  the  experiments,  and  in  the  interpretation  of  the  data, 
has  in  no  small  way  contributed  to  the  preparation  of  this  report. 
Mr.  M.  R.  Miller  and  Mr.  W.  C.  Matthews  have  added  much  of  value 
in  photographic  records.  A  number  of  the  county  horticultural  com- 
missioners, their  deputies  and  inspectors,  have  freely  given  advice 
and  both  their  time  and  transportation  facilities  whenever  needed. 
Mr.  Fred  Lowrie  and  Mr.  George  Lowrie  have  generously  allowed  the 
use  of  land  and  a  pumping  plant  for  experimental  purposes. 


*  Circular  168  of  this  station,  ' '  Spraying  for  the  control  of  the  wild  morning- 
glory  within  the  fog  belt,"  is  now  available  for  distribution  and  may  be  had 
upon  application  to  the  director. 


70  University  of  California  Publications  in  Agricultural  Sciences     [Vol.  4 

Mr.  F.  E.  Sullivan,  Mr.  Geo.  T.  Scott,  and  other  officials  of  the 
Spreckels  Sugar  Company  have  shown  a  hearty  spirit  of  cooperation 
and  have  made  possible  the  accumulation  of  many  valuable  data  by 
Mr.  A.  M.  Hunt  and  Mr.  H.  K.  Fox,  to  whom  was  assigned  the  execu- 
tion of  the  detail  of  the  cooperative  experiments  in  the  Salinas  Valley. 

SCOPE   OF   INVESTIGATIONS 

The  wild  morning-glory  is  undoubtedly  the  most  difficult  to  con- 
trol of  any  of  the  weed  pests  of  the  state.  It  is  widely  distributed  and 
is  rapidly  spreading.  The  known  methods  of  control  are  probably 
the  least  satisfactory  of  those  for  any  weed.  The  greatest  need  for 
investigation  appeared  to  be  the  control  of  this  pest.  The  principal 
part  of  the  problem  attacked,  therefore,  was  the  control  of  wild 
morning-glory  upon  agricultural  land  without  injury  to  the  soil. 

METHODS  OF  USING  HERBICIDES 

Chemicals  have  been  used  for  the  control  of  weeds  in  at  least  two 
ways  in  which  the  manner  of  action  of  the  poison  on  the  plant  is 
fundamentally  different.  A  well  defined  conception  of  this  difference 
in  principle  is  essential  to  the  formulation  of  control  measures.  One 
or  the  other  may  be  the  more  suitable  procedure  to  adopt,  depending 
on  various  conditions.  The  experiments  hereafter  reported  include 
tests  of  these  two  methods,  which,  for  the  purpose  of  discussion  will 
be  referred  to  as:  the  "root-absorption  method,"  and  the  "leaf- 
absorption  method.' ' 


TESTS   OF  HERBICIDES  BY  ROOT-ABSORPTION  METHOD 

Arsenic,  sodium  cyanide,  sulfuric  acid,  acid  sludge,  acid  tar,  salt, 
carbon  bisulfide,  copper  sulfate,  and  iron  sulfate  were  tested  against 
wild  morning-glory  by  the  root-absorption  method.  These  were  ap- 
plied so  that  the  soil  in  the  vicinity  of  the  roots  of  the  weeds  was 
more  or  less  permeated  with  the  herbicide.  In  this  method  the  poisons 
used  per  square  yard  varied  from  one-half  an  ounce  to  three  and  one- 
half  pounds  in  testing  some  materials.  Experimental  plots  were 
Located  a1  Whittier,  Centerville,  and  at  Spreckels. 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  71 


AT  WHTTTIER  AND  CENTERVILLE 

The  first  experiments  were  made  at  Whittier,  September  10,  1915, 
but  conclusions  could  not  be  drawn  on  account  of  the  short  duration 
of  the  observations.  They  did  indicate,  however,  that  sodium  arsenite 
and  sodium  cyanide  were  worthy  of  a  further  trial  against  morning- 
glory  and  Johnson  grass,  and  that  salt  applied  at  the  rate  of  3% 
pounds  per  square  yard,  under  the  conditions  of  the  experiment,  was 
of  no  value  against  the  above  weeds. 

Experiments  were  made  later  on  the  Lowrie  Ranch  at  Centerville, 
designed  principally  to  test  the  effect  of  the  root  absorption  of  four 
materials  on  morning-glory  and  the  effect  of  these  materials  on  the 
soil. 

Experimental  Plots. — The  land  assigned  for  the  experimental  plots 
was  level,  and  uniformly  and  heavily  infested  with  wild  morning- 
glory,  the  vines  forming  a  dense  mat,  in  most  places,  four  or  five  inches 
thick.  The  soil  was  visible  in  a  few  spots  only.  No  other  vegetation 
was  growing  on  the  land  at  the  time  of  the  treatment.  A  crop  of 
garlic  had  been  grown  previously  on  the  land  and  harvested  in  May 
or  June,  1915. 

The  land  was  marked  off  into  rows  1  yard  wide  and  10  yards  long, 
each  row  being  divided  into  ten  plots  of  a  square  yard  each.  The  rows 
were  lettered  A,  B,  C,  etc.,  and  the  plots  of  each  row  numbered  1  to  10, 
inclusive. 

Chemicals  Tested. — On  October  9,  1915,  different  treatments  were 
made  on  93  plots.    The  four  materials  tested  were : 

1.  Arsenic  in  the  form  of  sodium  arsenite:  a  stock  solution  was 
prepared  by  dissolving  arsenic  trioxide  in  sodium  hydroxide  and  water 
in  such  proportions  that  each  gallon  contained  the  equivalent  of  4 
pounds  of  arsenic  trioxide.* 

2.  Sodium  cyanide :  a  stock  solution  was  prepared  by  dissolving 
the  solid  in  water  and  diluting  so  that  each  gallon  contained  2  pounds 
of  sodium  cyanide. 

3.  Sulfuric  acid  (commercial  66°  Baume)  :  a  powerful  corrosive  to 
vegetable  tissue. 

*  The  stock  solution  formula  is : 

White  arsenic  (arsenic  trioxide  99%)  20  pounds 

Granulated   caustic    soda    (98%)    10  pounds 

Water,   to   make   5  gallons 

Detailed  directions  for  the  preparation  of  the  above  stock  solution  are  given 
in  Circular  168  of  this  station,  Spraying  for  the  control  of  wild  morning- 
glory  within  the  fog  belt. 


72  University  of  California  Publications  in  Agricultural  Sciences         [Vol.4 

4.  Acid  sludge :  until  recently  a  waste  product  in  the  refining  of 
petroleum  distillates  with  sulfuric  acid.  Previous  unpublished  experi- 
ments of  Mr.  E.  R.  de  Ong  and  the  writer  to  study  the  action  of 
petroleum  oils  on  citrus  and  other  foliage  had  indicated  that  the  con- 
stituents of  petroleum  distillates  which  are  capable  of  being  removed 
by  refining  with  sulfuric  acid  are  much  more  toxic  to  foliage  than  other 
constituents.  It  was  thought  that  acid  sludge,  therefore,  containing 
these  highly  toxic  constituents  of  petroleum  as  well  as  an  excess  of 
sulfuric  acid,  might  prove  to  be  an  efficient  and  economical  herbicide. 

Manner  of  Application. — For  each  treatment  a  quantity  of  the 
stock  solution  or  liquid  was  measured  out  and  diluted  to  a  volume  of 
either  one  or  two  gallons.  All  of  the  diluted  solution  was  then  uni- 
formly distributed  over  one  of  the  square-yard  plots  by  means  of  an 
ordinary  garden  sprinkling  pot.  It  was  found  by  preliminary  trial 
that  a  gallon  of  water  applied  to  a  square-yard  area  of  the  soil  would 
thoroughly  soak  the  ground  for  about  an  inch  below  the  surface.  This 
quantity  was  therefore  applied  to  some  of  the  plots;  on  others  the 
amount  of  liquid  was  doubled  in  order  to  ascertain  whether  or  not  the 
greater  amount  of  liquid  would  result  in  permeating  the  soil  with  the 
poisons  to  a  greater  depth  than  the  smaller  amounts.  It  was  thought 
that  the  vines  might  affect  the  absorption  of  the  poisons,  so  the  vines 
were  removed  from  some  of  the  plots  before  applying  the  solutions. 

Table  1  illustrates  in  detail  the  treatment  given  the  individual 
plots. 


AT  SPRECKELS 

Cooperative  experiments  with  the  Spreckels  Sugar  Company  were 
started  in  March,  1916,  on  its  ranches  in  the  Salinas  Valley. 

The  materials  tested  on  morning-glory  were :  iron  sulfate,  copper 
sulfate,  acid  sludge,*  acid  tar*  (derived  from  acid  sludge),  sulfuric 
acid,  carbon  bisulfide,  and  ^Nonpariel"  (a  commercial  herbicide,  con- 
sisting principally  of  carbon  bisulfide). 

Application  of  the  materials  was  made  on  square-yard  plots  in  the 
same  manner  as  in  the  Centerville  experiments  with  the  exception  of 
the  carbon  bisulfide  and  Nonpariel.  These  were  applied  by  making 
holes  on  the  plots  at  intervals  will)  an  iron  rod  and  pouring  into  the 
holes  a  quantity  of  the  material  and  immediately  filling  the  holes  with 
earth. 


"Furnished  by  courtesy  of  the  Union  Oil  Company  of  California. 


1919] 


Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds 


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University  of  California  Publications  in  Agricultural   Sciences     [Vol.  4 


Conclusions  Concerning  Root- Absorption  Method 

Inasmuch  as  the  root-absorption  method  gave  little  promise  of  offer- 
ing a  solution  of  the  morning-glory  problem,  space  will  not  be  taken 
for  a  detailed  report  of  the  results.  Table  2  will  be  of  interest  in 
showing  the  cost  of  the  more  effective  materials  tested.  Costs  shown  in 
this  table  and  elsewhere  in  this  report  are  calculated  from  the  retail 
prices  prevailing  at  the  time  of  the  treatments.*  The  cost  of  solids 
(arsenic  tri oxide  and  sodium  cyanide)  is  given  in  ounces  avoidupois; 
the  cost  of  liquids  (sulfuric  acid  and  carbon  bisulfide)  is  given  in 
fluid  ounces,  using  the  figure  1.8  as  the  specific  gravity  of  commercial 
sulfuric  acid,  and  1.27  as  that  of  carbon  bisulfide. 


TABLE  2 
Cost  of  More  Effective  Materials  Tested  by  Eoot- Absorption  Method 

Arsenic  Sodium  Sulfuric  Carbon 

trioxide  cyanide  acid  bisulfide 

Cost  per  ounce  as  applied,  approximate....     $0,008         $0,020         $0.0025  $0,006 
Least    amount    per    square    yard    giving 
any   promise    of    control    of    morning- 
glory,  ounces  3                   3                 20  10 

Cost  per  square  yard  $0,024         $0.06           $0.05  $0.06 

Cost  per  acre  117.90         290.40         242.00  305.50 

Least  amount  per  square  yard  prevent-  Soil  not 

ing  the  growth  of  practically  all  vege-  injured 

tation    except    morning-glory    for    14  by  this 

months  on  undisturbed  soil,  ounces....        1                 12                24  material 

Cost  per  square  yard  $0,008         $0.24  $0.06  

Cost  per  acre  39.30      1,160.60         290.40  

Cost  of  the  materials  in  table  2  are  calculated  from  the  following  prices: 

Arsenic  trioxide,  @  8c  per  lb.;  sodium  cyanide,  @  32c  per  lb.;  carbon  bisulfide, 
@90c  per  gal.;  caustic  soda,  @  10c  per  lb.;  sulfuric  acid,  @  2c  per  lb. 

No  data  are  available  on  the  cost  of  acid  sludge  and  acid  tar.  Until 
their  recent  use  in  the  flotation  process  for  the  concentration  of  certain 
low  grade  ores,  they  were  both  waste  products  in  refining  petroleum 
distillates  and  had  very  little  commercial  value. 

Nonpariel  sells  at  about  the  same  rate  as  carbon  bisulfide  or 
cheaper. 

No  sweeping  conclusions  are  justified  from  the  results  of  the  experi- 
ments. Whether  or  not  the  results  would  be  duplicated  on  other  soils, 
or  under  different  climatic  conditions,  or  if  the  chemical  were  applied 
at  other  seasons  of  the  y>>-<\v  as  yet  can  not  be  predicated. 

*  These  prices  are  above  normal.  Since  the  preparation  of  the  manuscript, 
the  price  of  arsenic  has  advanced  to  2:;  cents  per  pound.  Other  materials  are 
also  higher  and  some  are  difficult  to  obtain  .-it  any  price.  The  present  unsettled 
condition  of  the  markel   does  not  warrant  n  revision  of  prices. 


1919 J  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  75 

CONTROL  OF  WILD  MORNING-GLORY 
None  of  the  root-absorption  experiments  seemed  to  point  the  way 
for  the  control  of  wild  morning-glory  on  agricultural  land  at  a  reason- 
able expense  and  without  serious  injury  to  the  soil.  In  the  experiments, 
and  in  all  cases  that  have  come  to  the  attention  of  the  writer,  where 
a  chemical  has  been  used  by  this  method,  the  cost  would  prohibit  its 
use  as  a  control  measure,  except  on  small  areas;  if  eradication  has  not 
been  accomplished,  the  results  have  not  justified  the  means.  Further- 
more, it  was  found  that  in  using  a  non-volatile  herbicide  (such  as  salt, 
arsenic,  sodium  cyanide,  sulfuric  acid,  etc.),  if  the  soil  is  sufficiently 


M 


Fig.  1. — The  effect  of  arsenic  on  the  soil  when  applied  by  the  root-absorption 
method  (see  table  1). 

"Incidental  to  the  main  object  of  the  experiments,  .  .  .  data  .  .  .  show  the 
superiority  of  arsenic  as  a  soil  sterilizer. ' ' 

permeated  with  the  chemical  to  destroy  the  roots  of  the  wild  morning- 
glory,  it  is  rendered  unfit  for  the  growing  of  crops  for  many  months.* 
Carbon  bisulfide,  on  the  other  hand,  is  a  volatile  herbicide  and  appears 
to  produce  no  injurious  effects  on  the  soil.  It  is  also  quite  effective 
against  morning-glory,  but  the  expense  of  application  and  the  cost  of 
material  are  both  high. 

SOIL  STERILIZATION 

Incidental  to  the  main  object  of  the  experiments,  the  control  of  wild 

morning-glory  on  agricultural  land,  data  have  been  obtained  which 

show  the  superiority  of  arsenic  as  a  soil  sterilizer  (compare  figures 

1,  2,  and  3).    While  arsenic  has  apparently  failed  to  actually  eradicate 


*  See  further  discussions  of  the  effects  of  arsenic  on  the  soil  on  pages  90-91. 


76 


University  of  California  Publications  in  Agricultural  Sciences         [Vol.  4 


the  wild  morning-glory,  using  as  high  as  a  pound  to  the  square  yard, 
it  can  be  used  as  a  soil  sterilizer  in  respect  to  a  great  variety  of  weeds. 
All  of  the  Centerville  plots  to  which  an  ounce  or  more  of  arsenic  tri- 
oxide  had  been  applied  per  square  yard  were  barren  of  all  vegetation, 
except  morning-glory,  for  fourteen  months  (fig.  1),  notwithstanding 
the  leaching  by  the  rains  of  two  winters.  The  minimum  cost  of  mate- 
rials for  the  production  of  this  result  was  less  than  one  cent  per  square 
yard  or  about  forty  dollars  per  acre.*  This  would  not  be  prohibitive 
for  the  prevention  of  weed  growth  on  gravelled  walks,  tennis  courts, 


h. 


Fig.  2. — The  effect  of  sodium  cyanide  on  the  soil  when  applied  by  the  root- 
absorption  method   (see  table  1). 

On  all  plots  receiving  six  ounces  or  less  of  sodium  cyanide  the  final  growth 
of  miscellaneous  weeds  was  fully  equal  to  that  on  the  checks.  The  smaller 
amounts  appeared  to  stimulate  the  growth. 

roadways,  fencerows,  or  in  other  places  where  soil  sterilization  is 
desired. 

One  significant  fact  in  connection  with  soil  sterilization  is  that  any 
amount  of  arsenic  in  excess  of  one  ounce  per  square  yard  appears  to 
have  been  a  waste  of  material  unless  future  observations  will  show  that 
larger  amounts  will  have  a  more  lasting  effect. 

The  experiments  have  furnished  a  basis  for  further  experiments  in 
the  problem  of  weed  control  on  railroad  right-of-ways.  This  matter 
has  already  received  considerable  attention  by  a  number  of  railroads, 
but  the  danger  of  poisoning  live  stock  has  had  a  restraining  influence 


Sec  fool  note  on  page  71. 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  77 

on  the  use  of  arsenic  for  weed  control  in  this  connection.  The  sug- 
gestion to  add  some  substance  to  the  herbicide  as  a  repellant  to  stock 
has  been  given  attention,  but  sufficient  progress  has  not  yet  been  made 
to  warrant  publication. 


TESTS  OF  HERBICIDES   BY  LEAF-ABSORPTION  METHOD 

The  preceding  experiments  are  in  sharp  contrast  to  those  reported 
below.    In  the  former,  an  attempt  was  made  to  poison  the  soil  in  the 


Fig.  3. — The  effect  of  sulfuric  acid  on  the  soil  when  applied  by  the  root- 
absorption  method  (see  table  1). 

vicinity  of  the  root  system ;  in  the  latter,  the  poisons  were  not  applied 
to  the  soil,  but  were  sprayed  on  the  foliage  of  the  weeds.  For  the  pur- 
pose of  discussion,  the  latter  method  will  be  referred  to  as  the  "leaf- 
absorption  method."  The  amount  of  poison  required  in  this  method 
is  very  small  so  that  even  though  a  very  poisonous  substance  is  used, 
the  quantity  will  be  so  evently  distributed  over  the  area  treated  that 
the  probability  of  injury  to  the  soil  is  reduced  to  the  minimum.  It 
has  been  demonstrated  that  wild  mustard,  wild  radish,  and  other 
annual  weeds  can  be  economically  controlled  in  the  grain  fields  of  the 
middle  west  by  spraying  with  a  solution  of  iron  sulfate  or  of  copper 
sulfate.1' 2  A  dilute  spray  of  sodium  arsenite  is  extensively  employed 
in  the  Hawaiian  Islands  for  the  control  of  weeds  on  sugar,  rubber,  and 


78  University  of  California  Publications  in  Agricultural  Sciences     [Vol.  4 

pineapple  plantations.4' 9' 10  Experiments  have  shown  that  certain  oil 
sprays  are  effective  for  the  control  of  wild  onion  and  wild  garlic  in 
Indiana  and  Ohio.7' 1X  Experiments  in  Australia  have  proved  the  effec- 
tiveness of  an  arsenical  spray  for  the  destruction  of  cacti.5  In  the 
literature  just  cited,  the  leaf-absorption  method  has  been  demonstrated 
to  be  effective  for  the  control  of  weeds  on  a  large  scale.  So  far  as 
known,  however,  this  method  has  not  been  tried  against  the  wild 
morning-glory,  a  persistent  and  deep-rooted  perennial  (figs.  4  and  5). 
Bioletti3  reports  finding  roots  of  this  weed  well  supplied  with  starch 


Fig.  4. — A   remarkably   heavy   infestation    of   wild    morning-glory   covering 
about  two  acres  in  the  center  of  a  beet  field. 

The  stand  of  vines  was  well  above  one's  knees. 

and  capable  of  producing  new  shoots  at  a  depth  of  fourteen  feet,  the 
smallest  piece  of  which  is  capable  of  growing  and  originating  a  new 
plant. 

PRELIMINARY  EXPERIMENT  AT   CENTERVILLE 

Following  the  tests  of  the  root-absorption  method  at  Centerville,  a 
test  was  made  at  the  same  place  of  the  leaf -absorption  method  so  much 
in  use  in  Hawaii.9 

Experimental  Plot. — The  plot  selected  for  the  experiment  was 
immediately  adjoining  the  plots  used  for  the  root-absorption  experi- 
ments, and  consisted  of  51%  square  yards,  quite  uniformly  and  heavily 
infested  with  morning-glory,  the  vines  forming  a  dense  mat  four  or 
five  inches  thick  in  most  places.  No  other  vegetation  was  growing  on 
the  plot  at  the  time  of  treatment.  A  crop  of  garlic  had  been  grown 
on  the  land  and  harvested  in  May  or  June,  1915.  The  plot  was 
designated  as  Spray  Plot  1. 


1919] 


Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds 


79 


Chemical  Tested. — A  dilute  arsenical  solution  was  prepared  by 
mixing  4  fluid  ounces  of  the  arsenic  stock  solution  (p.  71)  with  3  gal- 
lons of  water.  This  made  a  solution  roughly  equivalent  to  a  dilution 
of  the  stock  solution  1 :100,  or  4  pounds  of  arsenic  trioxide  per  100 
gallons.  The  three  gallons  contained  the  equivalent  of  2  ounces  of 
arsenic  trioxide. 


Fig.  5. — Vines  and  root  of  wild  morning-glory. 

Manner  of  Application. — The  solution  was  applied  November  5, 
1915,  to  the  morning-glory  vines  on  the  plot  by  means  of  a  bucket 
spray  pump.  Just  enough  of  the  spray  was  used  to  moisten  the  leaves 
and  stems  of  the  vines  only,  no  attention  being  given  to  the  few  bare 
spots  where  the  soil  was  visible.  The  3  gallons  of  solution  was  found 
sufficient  for  the  purpose,  the  whole  plot  of  51%  square  yards  thus 
receiving  only  2  ounces  of  arsenic  trioxide. 


80  University  of  California  Publications  in  Agricultural  Sciences         [Vol.  4 

Results. — One  week  after  the  application  of  the  spray  the  morning- 
glory  vines  were  found  completely  killed  to  the  ground,  but  the  roots 
appeared  to  be  normal.  A  similar  observation  was  made  a  week  later. 
Three  weeks  after  spraying,  however,  the  effects  observed  on  the  roots 
of  the  morning-glory  vines  were  remarkable.  The  usual  new  growth 
of  sprouts  from  the  roots  after  the  destruction  of  the  vines  had  not 
occurred.  The  roots  were  moldy  and  disintegrating  to  a  depth  of 
several  feet  below  the  surface.  Forty-five  days  after  spraying,  only 
nine  plants  had  produced  any  new  growth  above  ground  on  the  whole 
plot,  while  it  was  estimated  that  fully  six  hundred  plants  were  above 
the  surface  on  an  equal  area  of  the  adjoining  field  which  had  been 
recently  gone  over  with  the  weed  knife.  The  usual  winter  growth  of 
wild  grasses  and  other  weeds  was  well  started  in  the  vicinity,  the 
growth  of  which  on  the  sprayed  plot  was  fully  equal  to  that  on  the 
adjoining  plot. 

Comments. — This  experiment  indicated  that  the  leaf-absorption 
method  for  the  control  of  morning-glory  on  agricultural  land  has 
great  possibilities.  The  cost  of  materials  did  not  exceed  $1.50  per 
acre,*  the  poison  having  been  applied  at  the  rate  of  12  pounds  per 
acre ;  the  remedy  was  applied  as  a  spray,  which  is  probably  the  most 
economical  manner  of  application;  the  apparent  control  of  the  weed 
at  a  comparatively  trifling  expense  was  almost  equal  to  the  best  results 
obtained  in  any  of  the  root-absorption  method  experiments;  and  no 
ill  effects  to  the  soil  were  observed.  The  possible  utility  of  this  method 
was  also  noted  for  the  control  of  miscellaneous  weeds  upon  non-agri- 
cultural land  where  cultivation  is  difficult  or  impossible. 

In  view  of  the  results  obtained  in  the  first  test  of  the  leaf -absorp- 
tion experiment  it  seemed  desirable  to  make  as  thorough  a  study  as 
possible  of  this  method  and  to  secure  sufficient  land  for  the  purpose 
with  reasonable  assurance  that  the  experiments  would  not  be  molested. 

PRELIMINARY  EXPERIMENTS  AT  DAVIS 
A  series  of  experiments  at  the  University  Farm  at  Davis  was  out- 
lined and  work  started  before  the  occurrence  of  the  winter  frosts 
of  1915. 

Experimental  Plots. — Most  of  the  land  assigned  for  the  experi- 
ments was  well  infested  with  morning-glory.    It  had  been  used  in  the 
summer  of  1915  for  a  crop  of  vegetables.    No  plants  other  than  morn- 
ing-glory were  growing  on  the  plots  when  the  experiments  were  begun. 
The  land  to  be  used  for  our  tests  was  divided  into  thirty  plots; 


"  Bee  footnote  on  page  74. 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  81 

of  one-fortieth  of  an  acre  each.  The  experiments  were  outlined,  prin- 
cipally, to  determine  the  most  suitable  of  three  forms  of  soluble  arsenic 
and  the  time  of  the  year  to  obtain  the  best  absorption  of  these  mate- 
rials through  the  aerial  parts  of  the  weeds. 

Chemicals  Tested  and  Manner  of  Application. — On  December  23, 
1915,  one  of  the  plots  was  treated  in  exactly  the  same  manner  as 
described  in  the  preliminary  experiments  at  Centerville.  A  dilute 
arsenical  solution  was  prepared  by  mixing  91/3  fluid  ounces  of  the 
arsenic  stock  solution  described  on  page  71  with  7  gallons  of  water. 
Another  plot  was  sprayed  the  same  day  with  a  solution  of  the  same 
concentration  of  arsenic  but  made  up  with  half  the  quantity  of  caustic 
soda.  A  third  was  sprayed  the  following  day  with  a  solution  of 
arsenic  acid  containing  the  same  amount  of  actual  arsenic  as  the  two 
previously  described  sprays. 

Results  Complicated  the  Problem. — Repeated  observations  subse- 
quent to  the  application  of  these  sprays  at  Davis  failed  to  disclose 
any  injury  to  the  roots  of  the  morning-glory  vines  on  any  of  the 
plots.  The  striking  results  of  the  Centerville  experiment  and  the 
failure  of  similar  treatments  two  months  later  at  Davis  clearly  sug- 
gested the  importance  of  making  a  study  of  all  conditions  which  might 
affect  the  results.  It  was  thought  that  the  climate,  the  season  of  the 
year,  the  weather  at  or  near  the  time  of  application,  the  condition  of 
the  plants,  the  type  and  condition  of  the  soil,  individually  or  collec- 
tively, and  possibly  others,  might  be  factors  influencing  the  results 
to  a  greater  or  less  extent. 

PLAN  OF  EXPEEIMENTS   IN   1916 

The  problem  at  once  became  so  complicated  that  it  seemed  wise 
to  limit  the  future  experiments  to  a  study  of  the  leaf-absorption 
method,  and  to  pay  most  attention  to  making  a  thorough  test  of  the 
arsenical  spray  treatment  under  as  many  conditions  as  possible.  The 
winter  rains  and  the  killing  of  the  morning-glory  vines  by  the  frost 
prevented  any  further  experiments  until  March,  1916,  at  which 
time  new  ones  were  started. 

Comprehensive  series  of  experiments  were  carried  on  during  the 
growing  season  of  1916  and  until  the  vines  were  killed  by  the  frosts. 
The  killing  frosts  of  1916  occurred  earlier  than  those  of  1915  so  that 
the  experiments  had  to  be  discontinued  at  an  earlier  date.  The  Center- 
ville and  Davis  experiments  were  continued  and  cooperative  experi- 
ments in  the  Salinas  Valley  in  cooperation  with  the  Spreckels  Sugar 


82  University  of  California  Publications  in  Agricultural   Sciences     [Vol.  4 

Company  were  also  started,  as  well  as  a  series  of  experiments  at 
Berkeley. 

Experimental  plots  were  thns  located  on  different  types  of  soil 
in  different  localities,  upon  soil  of  the  same  type  but  differing  in 
drainage  conditions,  and  upon  both  lightly  and  heavily  infested  plots. 
The  weather  conditions  at  the  various  experimental  plots  were  differ- 
ent. The  climate  at  Davis  is  semiarid,  while  the  remainder  of  the 
plots  were  within  the  fog  belt,  although  differing  somewhat  in  dis- 
tance from  the  coast.  The  details  of  all  the  experiments  of  1916  will 
not  be  described  in  full  in  this  progress  report  but  only  such  observa- 
tions as  seem  to  be  the  most  significant. 

FURTHER  EXPERIMENTS  AT  CENTERVILLE 

The  experiments  at  Centerville  were  continued  on  more  infested 
land  adjoining  the  plots  previously  used. 

Outline  of  Experiments  and  Subdivision  of  Plots. — In  order  to 
make  observations  on  the  results  to  be  obtained  by  repeated  sprayings 
at  different  intervals,  Spray  Plot  1  was  divided  into  two  parts,  A  and  B. 
Spray  Plot  1A  was  further  divided  into  five  small  plots  (  1  by  5 
yards)  which  were  lettered  a,  b,  c,  d,  and  e;  certain  ones  to  be  re- 
sprayed  at  irregular  intervals,  depending  upon  the  growth  of  weeds. 

Spray  Plot  IB  to  be  resprayed  once  in  the  fall  of  1916. 

Convenient  sized  plots  (2  by  10  yards)  were  staked  out  adjoining 
Spray  Plot  1,  to  be  sprayed  at  different  times  of  the  year. 

The  manner  of  subdivision  and  the  dates  on  which  the  sprays  were 
applied  are  indicated  in  tables  3  and  4. 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds 


83 


TABLE    3 

Spray  Plots  1A  and  IB,  Showing  Dates  of  Spraying 


Sprayed 

Sprayed 

11/5/15 

10/14/16 

<^~ 

10/14/16 

r> 

^-^ 

a 

b 

c 

d 

e 

Sprayed 

Sprayed 

Sprayed 

Sprayed 

Sprayed 

11/5/15 
3/11/16 

11/5/15 
3/11/16 

11/5/15 
3/11/16 

11/5/15 
3/11/16 

11/5/15 

5/5/16 

5/5/16 

5/5/16 

6/16/16 

6/16/16 

7/15/16 

7/15/16 

8/15/16 
10/14/16 

10/14/16 

10/14/16 

10/14/16 

10/14/16 

"\ 

^> 

84  University  of  California  Publications  in  Agricultural  Sciences         [Vol.  4 

TABLE  4 

Spray  Plots  2  to  7 

South  of  and  Adjoining  Spray  Plot  1.    Plots  2  by  10  Yards  Checks 
1  by  10  Yards.    Showing  Dates  of  Sprayings 


E 


U 


w 


Check 

Sprayed  10/14/16 

Spray  Plot  2 

Sprayed  3/11/16  and  10/14/16 

Spray  Plot  3 

Sprayed  6/16/16  and  10/14/16 

Check 

Sprayed  10/14/16 

Spray  Plot  4 

Sprayed  7/15/16  and  10/14/16 

Spray  Plot  5 

Sprayed  8/15/16  and  10/14/16 

Check 

Sprayed  10/14/16 

Spray  Plot  6 

Sprayed  9/16/16  and  11/11/16 

Spray  Plot  7 

Sprayed  10/14/16 

Check 

Sprayed  11/11/16 

Garlic 
field 
in  1915 


Tomato 
field 
in  1916 


1919] 


Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds 


85 


Chemicals  Tested  and  Manner  of  Application. — The  sprays  were 
all  made  up  and  applied  in  the  manner  described  in  the  preliminary 
experiment  at  Centerville.  Four  fluid  ounces  of  the  arsenic  stock  solu- 
tion (p.  71)  were  mixed  with  3  gallons  of  water.  The  solutions  were 
applied  by  means  of  a  pressure  sprayer,  using  just  enough  to  moisten 
the  foliage  of  the  weeds.    The  quantity  required  for  the  purpose  was 


SURFACE, 


Fig.  6. — Typical  root  systems  of  morning-glory  plants  at  Centerville,  de- 
stroyed by  an  arsenical  spray  applied  to  the  aerial  parts  only. 

Spray  applied  September  16,  1916;  roots  dug,  October  14,  1916.  Eoot  1  was 
dissected  out  to  a  distance  of  4  feet,  2  inches  below  the  surface  and  was  one- 
half  inch  in  diameter  at  D. 

roughly  equivalent  to  300  gallons  per  acre,  containing  sodium  arsenite 
equivalent  to  12  pounds  of  arsenic  trioxide,  and  6  pounds  of  sodium 
hydroxide.  The  cost  of  materials  for  the  300  gallons  of  spray  was 
estimated  to  be  $1.50  at  retail  price  prevailing  at  the  time  of  treat- 
ment.* 


*  See  footnote  on  page  74. 


86 


University   of   California  Publications  in  Agricultural   Sciences     [Vol.4 


Results. — Each  spraying  entirely  killed  the  vines  of  the  morning- 
glory  to  the  surface  of  the  ground,  as  well  as  most  of  the  broad-leafed 
plants.  The  grasses  were  much  less  affected.  The  roots  of  the  morn- 
ing-glory were  not  materially  injured  by  any  spray  applied  between 
March  and  August,  1916,  but  the  spray  applied  August  15,  1916, 
caused  marked  injury  to  the  roots  of  the  morning-glory.  The  usual 
growth  of  new  sprouts  was  markedly  less  than  usual,  only  18  having 
reached  the-  surface  within  one  month  after  spraying.  Even  these 
seemed  to  be  lacking  in  vitality.  Many  roots  were  apparently  dead 
from  iy2  to  2  feet  below  the  surface  of  the  ground. 

The  effect  of  the  spray  applied  September  16,  1916,  was  still  more 
encouraging.     One  month  after  the  application  of  the  spray,  a  hole 


Fig.  7. — Showing  the  injured  condition  of  the  interior  of  a  section  of  the 
lower  end  of  root  1,  figure  6,  four  feet  below  the  surface. 


was  dug  on  the  plot  to  a  depth  of  five  feet.  Working  laterally  from 
this  hole,  largely  by  means  of  trowel  and  geology  pick,  the  soil  was 
removed  from  some  twenty-five  roots  (see  fig.  6).  Only  one  of  these 
roots  appeared  to  be  normal.  The  rest  were  shrivelled,  moldy,  and 
disintegrated  from  the  base  of  the  vine  stems  to  two  or  three  feet 
below  the  surface,  the  remainder  still  being  plump.  On  exposing  the 
interior  of  these  plump  lower  sections  of  the  roots,  the  cambium  was 
found  to  be  turned  to  a  chocolate  brown  color  and  a  very  pronounced 
"sour"  odor  was  given  off.  Figure  7  will  give  an  idea  of  the  appear- 
ance of  the  interior  of  a  section  taken  from  the  lower  end  of  root  1, 
figure  6,  four  feet  below  the  surface. 

October  Sprayings  Most  Effective. — All  plots  previously  sprayed 
(with  the  exception  of  Spray  Plot  6)  were  sprayed  again  October  14, 
1916,  and  the  check  plots  between  were  also  sprayed  in  order  to  make 
observations  on  the  efficiency  of  two  sprayings  at  different  intervals 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  87 

and  to  make  the  attempt  to  disinfest  all  the  experimental  plots.  The 
October  spraying  appeared  to  be  more  effective  than  any  previous 
one. 

On  November  11,  1916,  a  count  was  made  of  the  morning-glory 
plants  producing  new  vines  on  the  various  plots  and  their  condition 
was  noted  with  the  results  shown  in  table  5. 

TABLE  5 
Observations  on  Centerville  Plots 

Observations,   Nov.   11,    1916 
Spray  plot  Number  of        Condition  of 

No.  When  sprayed  vines  vines 

1  Plot  IB  Nov.,  1915,  Oct.,  1916  27  Sickly 

Plot  1A  (see  map,  p.  83). 

Check  October,  1916  0 

2  March  and  October,  1916  8  Sickly 

3  June  and  October,  1916  10  Sickly 

Check           October,  1916  3  Sickly 

4  July  and  October,  1916 5  Sickly 

5  August  and  October,  1916  40  Vigorous 

Check           October,    1916    0 

6  September  only,  1916  34  Sickly 

7  October  only,  1916  5  Sickly 

TOTAL  132 

Total  area  included  in  the  above  count  200  sq.yds. 

Estimated  number  of  plants  on  this  area  before  treatment  2,400 

Total  number  of  plants  producing  new  vines  on  this  area  after 

treatment    132 

Numbered  plots,  2  by  10  yards;  checks,  1  by  10  yards. 


Resume  of  Leaf-Absorption  Experiments 

The  experiments  at  Davis  were  continued  during  1916  until  the 
vines  were  killed  by  the  frost  early  in  November.  The  three  arsenical 
sprays  (p.  81)  were  applied  to  additional  plots  in  the  months  of 
March,  May,  June,  July,  August,  September,  and  October.  None  of 
the  sprayings  produced  very  satisfactory  results.  A  comparison  of 
the  efficiency  of  the  three  forms  of  arsenic  is,  therefore,  impossible. 
In  some  of  the  experiments  the  arsenic  acid  spray  seemed  to  be  more 
severe  in  its  action  than  others. 

The  Spreckles  Sugar  Company  has  provided  for  the  full  time  of  one 
man  since  March,  1916,  for  cooperative  experiments  on  their  ranches 
in  the  Salinas  Valley.  Arsenical  sprays  of  different  concentrations 
were  applied  to  morning-glory  every  month  of  the  year  from  March 
to  December.     The  problem  of  the  control  of  miscellaneous  weeds 


88  University  of  California  Publications  in  Agricultural  Sciences        [Vol.  4 

along  their  extensive  irrigation  system  and  along  roadways  was  also 
attacked.  Arsenic  was  given  the  most  extensive  trial,  and,  in  addition, 
sodium  cyanide,  iron  sulfate,  copper  sulfate,  acid  sludge,  and  acid 
tar  were  tested. 

A  number  of  experiments  were  also  made  at  Berkeley,  testing  the 
arsenical  spray  on  morning-glory. 

Each  set  of  experiments  has  contributed  materially  in  throwing 
light  on  the  problem  of  the  control  of  weeds  by  means  of  chemicals. 
The  results  of  the  above  experiments  are  summarized  under  the  various 
headings  below. 


PETEOLEUM  PRODUCTS  AS  HERBICIDES 

While  arsenic  was  found  to  be  not  only  the  most  potent  of  the 
plant  poisons  thus  far  tested,  but  also  the  cheapest,  there  are  certain 
dangers  attending  its  use  which  must  not  be  overlooked.  This  fact 
has  led  the  writer  to  make  a  search  for  an  herbicide  which  is  less 
poisonous  to  man  and  animals,  and  yet  not  prohibitive  in  price.  The 
experiments  in  this  direction  were  rather  limited  but  were  sufficiently 
encouraging  to  warrant  further  investigation. 

A  spray  composed  of  equal  parts  of  acid  sludge  (p.  72)  and  water, 
applied  to  wild  morning-glory  vines  in  October,  1916,  appeared  to  be 
as  effective  as  arsenic  in  destroying  the  weed.  Acid  tar(  a  derivative 
of  acid  sludge  containing  less  free  acid)  gave  encouraging  results 
on  morning-glory. 

The  above  materials  applied  as  a  spray,  either  pure  or  diluted, 
were  especially  effective  on  grasses.  A  spray  of  acid  tar  applied  to 
a  plot  of  succulent  grass  (principally  foxtail  and  wild  oats)  caused 
withering  and  browning  of  the  leaves  within  half  an  hour  after  appli- 
cation; within  twenty-four  hours  the  grass  readily  took  fire  from  a 
lighted  match  and  burned. 

Petroleum-distillate  sprays  were  tested  on  miscellaneous  weeds. 
The  results  indicated  that  certain  of  these  distillates  were  more  effec- 
tive in  destroying  grasses  than  sodium  arsenite,  while  sodium  arsenite 
was  generally  more  effective  on  the  broad-leafed  plants  than  the  distil- 
lates. The  cheaper  grades  of  distillates  were  more  effective  than  re- 
fined distillates  such  as  kerosene  and  similar  products.  Petroleum 
products  containing  a  large  percentage  of  aromatic  and  "cracked" 
oils  were  more  effective  than  those  containing  a  lesser  percentage. 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  89 

AKSENIC    ON   MISCELLANEOUS   ANNUAL   WEEDS 

The  growth  of  many  annual  weeds  was  easily  killed  to  the  ground 
by  spraying  with  a  dilute  arsenical  spray.  The  roots  were  not  always 
destroyed  and  would  often  produce  a  new  growth. 

Annual  weeds  of  more  tender  foliage  were  destroyed  with  a  spray 
containing  the  equivalent  of  four  pounds  of  arsenic  trioxide  per  hun- 
dred gallons.  The  grasses  were  more  resistant  to  arsenic  than  most 
other  weeds.  These  and  weeds  of  more  hardy  foliage  seemed  to  re- 
quire a  more  concentrated  spray  than  the  above.  Sprays  containing 
the  equivalent  of  eight  to  twenty  pounds  of  arsenic  trioxide  per  hun- 


;i 


Fig.  8. — The  effect  of  arsenic  on  the  soil  when  applied  by  the  leaf  absorp- 
tion method  (see  table  3). 

Spray  Plot  1A  had  been  sprayed  twice,  November  5,  1915,  and  March  11,  1916 ; 
and  Spray  Plot  IB  had  been  sprayed  once,  November  5,  1915,  when  the  photo 
was  taken,  April  28,  1916.'  The  growth  of  miscellaneous  weeds  on  the  plots  after 
treatment  has  been  used  throughout  the  experiments  as  an  indication  of  the 
effect  of  the  treatments  on  the  soil. 

dred  gallons  have  usually  given  satisfactory  results  in  the  control 
of  the  more  resistant  annual  weeds.  The  optimum  concentration  was 
not  determined,  but  it  appears  that  it  will  have  to  be  varied  according 
to  the  nature  of  the  weeds. 

The  arsenical-spray  method  has  been  shown  to  be  suitable  for  the 
control  of  miscellaneous  annual  weeds  growing  along  irrigation  ditches, 
f encerows,  and  other  places  where  cultivation  is  difficult  or  impossible 
and  for  the  prevention  of  the  maturing  of  seeds  of  perennial  noxious 
weeds  on  uncultivated  land. 


90 


University   of  California  Publications  in  Agricultural   Sciences     [Vol.4 


Soap  as  a  Spreader. — The  addition  of  three  or  four  pounds  of  soap 
(first  dissolved  in  hot  water)  to  each  hundred  gallons  greatly  increased 
the  effectiveness  of  the  arsenical  sprays.  This  addition  was  found 
to  be  especially  desirable  when  the  sprays  were  used  against  the 
grasses  or  other  waxy-coated  foliage  upon  which  the  spray  had  a 
tendency  to  collect  in  drops. 

ARSENIC  ON  WILD  MORNING-GLORY 

Effect  on  the  Soil, — The  germination  and  growth  of  the  natural 
weed  seeds  on  the  plots  after  treatment  has  been  used  throughout 


.«. 


'.'•■.'■•- 


Fig.  9. — The  effect  of  arsenic  on  the  soil  when  applied  by  the  leaf  absorption 
method   (see  table  3). 

A  closer  view  of  Spray  Plot  IB.  The  luxuriant  growth  of  weeds  subsequent 
to  spraying  is  good  evidence  of  the  non-injurious  effect  of  arsenic  on  the  soil 
when  applied  by  the  leaf  absorption  method. 

the  experiments  as  an  indication  of  the  effect  of  the  treatments  on  the 
soil.  The  luxuriant  growth  of  weeds  shown  in  figures  8  and  9  is 
good  evidence  of  the  non-injurious  effect  of  arsenic  on  the  soil  when 
applied  by  the  leaf-absorption  method.  While  arsenic  has  been  shown 
to  be  very  toxic  to  the  soil  when  applied  in  large  quantities,  as  in  the 
root-absorption  method,  the  amount  required  by  the  leaf-absorption 
method  is  so  small  that  no  ill  effects  to  the  soil  could  be  detected. 
Even  six  successive  applications  of  the  spray  failed  to  reveal  any 
indication  of  deleterious  effects. 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  91 

A  study  of  the  effect  of  arsenic  on  the  soil  organisms  that  produce 
nitric  nitrogen  has  been  made  by  Professor  W.  F.  Gericke,  of  the 
Division  of  Soil  Chemistry  and  Bacteriology  of  this  station.  So  far 
as  this  important  function  of  soil  fertility  is  concerned,  this  study  has 
shown  that  an  amount  of  arsenic  equivalent  to  one  hundred  successive 
applications  of  the  spray,  as  applied  to  the  Centerville  plots,  produced 
no  toxic  effect  on  the  nitrifying  bacteria.  It  is  evident,  however,  that 
the  functions  of  the  soil  are  interfered  with  when  large  quantities 
of  arsenic  are  applied.  In  the  root-absorption  experiments,  the  above 
amount  of  arsenic  rendered  the  soil  barren  of  all  vegetation,  except 
morning-glory,  for  many  months. 

The  writer  is  indebted  to  Professor  Gericke  for  the  following  con- 
tribution concerning  the  effect  of  arsenic  on  the  flora  of  the  soil : 

TABLE   6 

Nitrification  of  Soil  from  Centerville 

Treated  with  sodium  arsenite  to  kill  weeds 


Laboratory 
number 

1 

Plot 
number* 

A-l 

Depth  of  sample 
in  inches 

0-6 

As  2Os  applied 

to  plots  in 

the  form  of 

sodium  arsenite 

oz.  per  sq.  yd. 

16 

Mg.  of  nitric 
nitrogen  per 
100  gms.  soil. 

Duplicates 
(1)            (2) 

2.3           4.0 

Average  of 
determinations 
3.16 

2 

A-l 

12-24 

16 

20.0 

14.0 

17.00 

3 

A-3 

0-6 

8 

18.0 

18.0 

18.00 

4 

A-3 

12-24 

8 

10.4 

12.0 

11.20 

5 

A-7 

0-6 

4 

24.8 

24.0 

24.40 

6 

A-7 

12-24 

4 

26.0 

26.8 

26.40 

7 

A-10 

0-6 

1 

24.8 

26.0 

25.40 

8 

A-10 

12-24 

1 

24  0 

20.0 

22.00 

9 

E-2 

0-6 

0 

20.0 

20.0 

20.00 

10 

E-2 

12-24 

0 

22.0 

20.0 

21.00 

*  See  table  1,  page  73. 

The  data  in  table  6  show  that  the  sodium  arsenite  applied  at  the  rate  of  the 
equivalent  of  8  ounces,  or  more,  of  arsenic  trioxide  per  square  yard  was  toxic 
to  the  soil  organism  that  produced  nitrification.  Toxic  effects  were  noted  in 
both  surface  and  subsoil  in  these  larger  applications.  On  the  plots  in  which 
the  sodium-arsenite  application  was  equivalent  to  4  ounces  or  less  of  arsenic 
trioxide  per  square  yard,  no  toxic  effect  was  noted;  on  the  contrary,  the  results 
indicate  stimulation  of  the  soil  organism  that  produce  nitric  nitrogen.  The 
results  are  in  accord  with  the  data  published  by  Greaves^  who  reported 
stimulating  effects  to  ammonification  and  nitrification  in  soil  to  which  small 
amounts  of  certain  arsenic   compounds  were  added. 

Concentration  of  Spray. — Different  concentrations  of  arsenical 
sprays  were  tried  at  Spreckels,  varying  from  the  equivalent  of  four 
ounces  to  sixteen  pounds  of  arsenic  trioxide  per  hundred  gallons. 
The  concentration  of  arsenic  used  throughout  the  Centerville  experi- 


92  University  of  California  Publications  in  Agricultural  Sciences        [Vol.  4 

ments  (four  pounds  of  arsenic  trioxide  per  hundred  gallons)  appeared 
to  be  as  effective  for  the  control  of  morning-glory  as  any  other  tested. 
Much  lower  concentrations  were  ineffective.  Higher  concentrations 
appeared  to  produce  no  better  results  and  possibly  not  as  good.  The 
stronger  sprays  in  some  instances  appeared  to  collapse  the  tissues  of 
the  vines  and  stop  the  circulation  of  the  sap  before  the  poison  had  its 
full  effect  upon  the  roots. 

Repeated  Applications  of  Small  Doses. — Repeated  small  doses  of 
arsenic  were  sprayed  upon  morning-glory  vines  at  intervals  of  one 
day,  four  days,  and  longer,  depending  upon  the  growth  of  the  new 
vines.  These  experiments  were  tried  in  the  spring  of  1916  to  ascertain 
if  the  effects  of  arsenic  applied  in  small  doses  would  be  cumulative. 
None  of  these  treatments,  however,  appeared  to  injure  the  roots  in 
any  way,  although  the  experiments  were  continued  well  into  the 
summer  months. 

Absorbing  Surface. — The  observation  has  been  made  many  times 
that  the  roots  of  morning-glory  plants  without  a  good  vine  develop- 
ment (that  is,  with  only  a  small  absorbing  surface)  were  not  severely 
injured  by  the  application  of  a  poisonous  spray  to  the  vines ;  and  that 
parts  of  the  plants  beneath  the  surface  at  the  time  of  spraying  would 
make  a  normal  growth  of  vines  subsequently,  if  not  connected  with  a 
part  which  did  have  a  good  growth  of  vines.  This  observation  con- 
tributes important  evidence  not  only  of  the  absorption  of  poisons 
through  the  aerial  parts  of  wild  morning-glory,  but  also  of  the  non- 
injurious  effect  of  the  arsenical  spray  on  the  soil. 

The  broad-leafed  weeds  were  generally  more  affected  by  arsenical 
sprays  than  the  grasses.  The  effect  of  arsenic  upon  foliage  (especially 
on  grasses  and  other  waxy-coated  foliage)  was  more  severe  if  soap 
was  added  to  the  spray  to  act  as  a  spreader. 

The  Time  of  the  Year. — The  arsenical  sprays,  excepting  a  few  very 
dilute  ones,  completely  killed  the  vines  of  the  wild  morning-glory  at 
whatever  time  of  the  year  they  were  applied. 

The  earliest  time  at  which  any  of  these  sprays  were  at  all  injurious 
to  the  roots  was  in  May,  at  Davis,  where  the  weed  matured  earlier  than 
on  any  of  the  other  experimental  plots.  At  this  time  many  of  the 
plants  were  in  the  green  seed-pod  stage.  All  sprayings  at  Davis  from 
May  to  October  produced  at  least  slight  injury  to  the  roots,  appar- 
ently depending  on  weather  condition,  but  none  of  them  was  con- 
sidered a  satisfactory  control. 

The  spring  and  early  summer  sprayings  on  all  of  the  other  experi- 


1919] 


Gray :  Tests  of  Chemical  Means  for  the  Control  of  Weeds 


93 


mental  plots  produced  little  or  no  injury  to  the  roots  of  the  weed, 
while  the  late  summer  and  fall  sprayings  produced  marked  injury  to 
the  roots  of  mature  vines.  The  amount  of  injury  apparently  increased 
as  the  season  advanced.  The  most  satisfactory  results  in  1915  were 
obtained  in  November;  in  1916,  in  October. 

The  establishment  of  this  point,  however,  was  not  a  solution  of  the 
problem,  as  will  be  shown  by  later  discussion. 

Maturity  of  Plants. — Observations  which  appear  to  have  an  impor- 
tant bearing  on  the  problem  are  as  follows : 


Fig.   10. — The  maturity  of  the  plant  influences  the  results. 

The  roots  of  some  of  the  morning-glory  plants  at  Centerville  were 
slightly  injured  by  an  arsenical  spray  applied  June,  1916,  and  by  one 
applied  a  month  later.  The  roots  of  those  plants  which  were  in  seed 
or  late  blossom  at  the  time  of  spraying  appeared  to  have  been  most 
severely  injured.  The  same  observation  has  been  made  repeatedly 
during  the  progress  of  the  other  experiments. 

A  spray  was  applied  to  mature  morning-glory  plants  at  Berkeley 
in  September.  The  roots  of  these  plants  were  completely  destroyed  by 
the  spray.  A  spray  applied  to  another  plot,  however,  under  prac- 
tically the  same  conditions,  failed  to  injure  the  roots.  It  was  observed 
that  in  the  latter  case  the  vines  were  actively  growing  at  the  time  of 
treatment,  since  they  had  been  cut  about  four  weeks  previously  with  a 
weed  knife.     One  plant,  however,  was  found  the  roots  of  which  were 


94 


University  of  California  Publications  in  Agricultural  Sciences     [Vol.  4 


moldy  and  disintegrated  as  far  as  dissected  out.  This  vine  apparently 
had  been  missed  by  the  weed  knife  and  was  much  older  than  the  other 
vines.  It  had  a  number  of  green  seed-pods  attached,  while  the  other 
vines  had  not  yet  blossomed.  The  two  disintegrated  roots  shown  at 
the  left  of  figure  10  were  taken  from  the  plant  mentioned  above  as 
being  in  seed  at  the  time  of  spraying ;  the  two  uninjured  roots  at  the 
right  of  the  figure  from  which  normal  new  sprouts  had  originated  were 
taken  from  another  plant  only  eight  inches  from  the  first.     So  far 


Fig.  11. — Typical  morning-glory  root  system  from  the  experimental  plots 
on  the  campus  at  Berkeley. 

The  entire  plant  was  in  a  state  of  disintegration  when  dug  three .  weeks 
after  spraying.  The  aerial  parts  originating  from  A  were  sprayed,  while  those 
originating  from  C  were  not  sprayed.  The  vines  originating  from  C  were  just 
beginning  to  wilt  three  weeks  after  the  application  of  the  spray  above  A. 

as  known,  the  general  climatic  conditions  and  local  weather  conditions 
at  the  time  of  spraying  were  practically  the  same  at  this  point  and 
on  the  plot  on  which  the  roots  were  entirely  destroyed.  Sprays  were 
applied  to  mature  vines  at  Berkeley  many  times  during  September 
and  October,  1916,  and  in  every  case  destroyed  all  signs  of  life  in 
both  vines  and  roots.  Identical  treatments  in  October  at  Centerville, 
a1  Spreckels,  and  in  Ventura  County  resulted  in  very  serious  injury 
to  the  roots  of  the  weed. 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  95 

Another  experiment  which  will  throw  light  on  the  same  matter  was 
made  at  Davis.  Two  plots  were  selected,  on  one  of  which  the  plants 
were  in  full  bloom  and  on  the  other  in  the  green  seed-pod  stage. 
Identical  sprays  were  applied  to  the  two  plots  on  the  same  day.  Sub- 
sequent observations  failed  to  reveal  any  injury  upon  the  roots  of  the 
plants  which  were  in  full  bloom  at  the  time  of  spraying.  The  roots 
on  the  plot  which  was  in  the  green  seed-pod  stage  were  seriously  in- 
jured by  the  spray  and  fully  75  per  cent  of  the  roots  failed  to  produce 
new  sprouts  within  a  month  after  the  time  of  spraying.  One  root 
dissected  out  from  this  plot  to  a  depth  of  three  feet  was  found  to  be 
moldy  and  disintegrated  throughout  its  entire  length. 

It  thus  appears  that  the  most  significant  difference  which  may 
account  for  apparent  conflicts  of  results  was  the  difference  in  the 
stage  of  maturity  of  the  plants.  The  remarkable  phenomenon  ob- 
served on  the  effect  on  the  root  systems  of  wild  morning-glory  vines 
produced  by  a  late  summer  or  fall  spraying  strongly  suggests  that 
the  appearance  of  this  phenomenon  is  intimately  associated  with  the 
rest  period  of  the  plant.  The  increased  downward  flow  of  the  sap 
of  plants  at  the  beginning  of  the  rest  period  is  well  known  to  plant 
physiologists  and  appears  to  be  a  most  significant  fact  in  this  con- 
nection. 

Influence  of  the  Weather. — Monthly  climatological  data  for  Cali- 
fornia have  been  received  through  the  courtesy  of  the  Weather  Bureau 
of  the  United  States  Department  of  Agriculture.  The  reports  from 
the  weather  station  nearest  the  various  experimental  plots  may  be 
taken  as  indicating  the  approximate  state  of  the  weather  at  or  near 
the  time  of  treatment.  In  many  cases  the  data  were  not  sufficiently 
local  or  complete  to  determine  accurately  to  what  extent  the  results 
of  the  experiments  were  influenced  by  the  weather.  From  a  general 
survey  of  the  weather  data  and  the  known  facts  concerning  the  influ- 
ence of  weather  on  foliage  injury  caused  by  insecticides  and  fungi- 
cides,14 it  seems  evident  that  the  most  important  weather  condition 
influencing  the  results  of  the  leaf -absorption  method  is  the  amount  of 
moisture  present  at  or  near  the  time  of  treatment.  The  encouraging 
results  obtained  by  the  leaf-absorption  method  experiments  in  all 
the  humid  coast  climates  and  the  unsatisfactory  results  obtained  by 
the  same  method  in  the  semiarid  climate  at  Davis  strongly  indicated 
that  one  essential  condition  of  the  successsful  application  of  arsenic 
as  an  herbicide  by  the  leaf -absorption  method  is  the  presence  of  suffi- 
cient moisture  to  prevent  the  drying  of  the  poison  on  the  leaves  before 
its  absorption. 


96  University  of  California  Publications  in  Agricultural  Sciences        [Vol-  4 

Although  none  of  the  experiments  at  Davis  was  wholly  satisfactory, 
even  though  applied  to  mature  vines  which  had  already  ripened  seeds, 
two  sprays  were  made  in  the  fall  which  quite  seriously  affected  the 
roots  of  the  morning-glory  vines.  In  both  cases,  the  root  injury 
occurred  from  the  sprayings  which  were  immediately  followed  by 
damp  weather. 

Possibilities  and  Limitations  of  the  Method. — The  method  described 
can  not  as  yet  be  said  to  be  one  of  eradication.  It  has  been  demon- 
strated, however,  that  85  to  90  per  cent  of  the  morning-glory  roots  on 
the  plots  near  the  coast  can  be  killed  to  a  depth  of  four  feet  or  more 
by  a  properly  timed  spray  to  mature  vines.  New  sprouts  will  emerge 
from  the  stubs  of  the  partially  killed  roots  and  will  eventually  reach 
the  surface  and  produce  new  vines.  Under  these  conditions  they  are, 
however,  very  puny,  the  leaves  being  only  about  one-fourth  of  the 
normal  size  and  of  a  sickly  yellowish  color.  The  new  growth  is  quite 
different  from  the  normal  trailing  vine.  When  the  new  shoot  reaches 
the  surface,  a  thick  clump  of  erect  branches  is  produced  not  more 
than  eight  or  nine  inches  in  length.  The  majority  of  the  new  shoots 
consume  from  seven  months  to  one  year  in  reaching  the  surface  so 
that  a  crop  can  be  well  established  on  the  land,  or  an  early  crop 
harvested,  without  serious  interference  from  the  weed. 

The  abnormal  condition  of  the  vines  originating  from  the  stubs  of 
the  partially  destroyed  roots  and  the  enfeebled  condition  of  these  root 
stubs  lead  one  to  believe  that  an  annual  fall  spraying  may  eventually 
eradicate  the  weed. 


SUMMARY 

None  of  the  root-absorption  experiments  seemed  to  point  the  way 
for  the  control  of  wild  morning-glory  on  agricultural  land  at  a  reason- 
able expense  and  without  serious  injury  to  the  soil. 

Incidental  to  the  main  object  of  the  experiments,  the  control  of 
wild  morning-glory  on  agricultural  land,  data  have  been  obtained 
which  show  the  superiority  of  arsenic  as  a  soil  sterilizer.  All  of  the 
Centerville  plots  to  which  an  ounce  or  more  of  arsenic  trioxide  had 
been  applied  per  square  yard  were  barren  of  all  vegetation,  except 
wild  morning-glory,  for  fourteen  months  notwithstanding  the  leaching 
by  the  rains  of  two  winters. 

The  leaf-absorption  experiments  have  definitely  established  one 
fact  of  importance  from  a  scientific  as  well  as  from  a  practical  stand- 


1919]  Gray:  Tests  of  Chemical  Means  for  the  Control  of  Weeds  97 

point,  namely,  that  a  dilute  solution  of  sodium  arsenite  applied  only 
to  the  aerial  parts  of  the  wild  morning-glory  under  certain  conditions 
will  destroy  both  the  aerial  parts  of  the  plant  and  the  underground 
parts  to  a  depth  of  several  feet. 

The  experiments  point  strongly  toward  the  conclusion  that  the 
accomplishment  of  these  results  is  chiefly  dependent  on  the  coexist- 
ence of  two  conditions  at  the  time  of  the  application  of  the  poison, 
namely  that : 

1.  The  plant  is  approaching  or  wholly  within  the  dormant  state. 

2.  Sufficient  moisture  is  present  in  the  air  to  prevent  the  drying 
of  the  poison  on  the  leaves  before  its  absorption. 


LITERATURE  CITED 

i  North  Dakota  Agr.  Exp.  Sta.,  10th  and  11th  Annual  Beports,  1900,  1901. 

2  Bolley,  H.  L.  North  Dakota  Agr.  Exp.  Sta.,  Press  Bulls.  9,  25,  26,  27,  28,  29, 
and  Bull.  80,  1903-1909. 

s  Bioletti,  F.  T.  The  extermination  of  morning-glory.  Calif.  Agr.  Exp.  Sta. 
Circ.  69,  1911. 

4  Wilcox,  E.  V.  Killing  weeds  with  arsenite  of  soda.  Hawaii  Agr.  Exp. 
Sta.,  Press  Bull  30,  1911. 

s  White  [White-Haney],  Jean.  Eeports  of  officer  in  charge  of  the  Prickly 
Pear  Experimental  Station,  Dulacca,  for  years  1912-1916.  Reprints  from  Ee- 
ports of  Queensland  Department  of  Public  Lands,  appendix  4  of  the  years, 
1912-1913,  1914,  1915. 

6  Greaves,  J.  E.  Some  factors  influencing  ammonification  and  nitrification  in 
soils.     I,  Influence  of  arsenic.    Centrbl.  f.  Bakt.,  2  Abt.,  Vol.  39,  pp.  542-560,  1913. 

7  Pipal,  F.  J.  Wild  garlic  and  its  eradication.  Purdue  Agr.  Exp.  Sta.  Bull. 
176,  1914. 

s  U.  S.  Dept.  Agr.  The  use  of  chemical  poisons  in  killing  weeds.  Weekly 
News  Letter  to  Crop  Correspondents,  vol.  2,  no.  7,  1914. 

&  McGeorge,  W.  T.  The  effect  of  arsenite  of  soda  on  the  soil.  Hawaii  Agr. 
Exp.  Sta.,  Press  Bull.  50,  1915.  Fate  and  effect  of  arsenic  applied  as  a  spray  for 
weeds.     Jour.  Agr.  Res.,  vol.  5,  p.  459,  1915. 

io  Krauss,  F.  G.  Suppression  of  weeds  among  pineapples  by  arsenite  of  soda 
spray.    Hawaii  Agr.  Exp.  Sta.,  Press  Bull.  48,  1915. 

11  Selby,  A.  D.,  and  Van  Atta,  D.  R.  Kill  garlic  and  wild  onion  by  oil 
spraying.     Mon.  Bull.  Ohio  Agr.  Exp.  Sta.,  vol.  1,  no.  12,  1916. 

12  Gray,  G.  P.  Herbicide  investigations.  California.  Cultivator,  vol.  46, 
no.  13,  1916;  Mon.  Bull.  Calif.  State  Comm.  Hort.,  vol.  5,  no.  4,  1916. 

is  Gray,  G.  P.  Spraying  for  the  control  of  wild  morning-glory  within  the 
fog  belt.     Calif.  Agr.  Exp.  Sta.  Circ.  168,  1917. 

14  Gray,  G.  P.  Lead  arsenates,  stone  fruits,  and  the  weather.  Jour.  Econ. 
Entom.,  vol.  10,  no.  4,  1917. 


